1. Field of the Invention
The present invention relates to an electro-luminescence display device, and more particularly to a method and apparatus for driving an electro-luminescence display device that is adaptive for increasing picture quality.
2. Description of the Related Art
Recently, there has been developed various flat display devices, which can be reduced in weight and bulk where a cathode ray tube CRT has a disadvantage. Such flat display panel includes a liquid crystal display, a field emission display, a plasma display panel, and electro-luminescence (hereinafter, EL) display device.
The structure and fabricating process of the PDP is relatively simple, thus the PDP is most advantageous to be made large-sized, but the light emission efficiency and brightness thereof is low and its power dissipation is high. It is difficult to make the LCD large-sizes because of using a semiconductor process, but since it is mainly used as a display device of a notebook computer, the demand for it increases, however there is a disadvantage that the LCD can hardly be made into a large-sized one and that power dissipation is high due to a backlight unit. Further, light loss by optical devices such as a polarizing filter, a prism sheet and diffusion plate is high and a viewing angle is narrow in the LCD. As compared with this, the EL display device is generally classified into an inorganic EL and an organic EL, and there is an advantage that its response speed is fast, its light-emission efficiency and brightness are high, and it has wide viewing angle. The organic EL display device can display a picture in a high brightness of several ten thousands [cd/m2] with a voltage of about 10[V].
In the organic EL display device, as shown in FIG. 1, there is formed an anode 2 of transparent conductive material on a glass substrate 1, and there are deposited a hole injection layer 3, a light-emission layer 4 of organic material, an electron injection layer 5 and a cathode 6 of metal on top of it. If an electric field is applied between the anode 2 and the cathode 6, holes in the hole injection layer 3 and electrons in the electron injection layer 5 respectively progress toward the light-emission layer 4 to be combined in the light-emission layer. Then, a fluorescent material in the light-emission layer 4 gets excited and transferred to generate a visible light. At this moment, the brightness is not proportional to a voltage between the anode 2 and the cathode 6 but is proportional to a current. Accordingly, an apparatus for driving the organic EL display device generally drives the organic EL display device by a constant current source.
Referring to FIG. 2, the apparatus for driving an organic display device of the related art includes a constant current source 21 applying current to data lines DL1 to DLm, and switching devices 22 and 23 applying a scan high voltage Vhigh and a ground voltage GND to each of scan lines SL1 to SLn.
The data lines DL1 to DLm act as the cathodes in FIG. 1, and the scan lines SL1 to SLn act as the anodes in FIG. 1. There are formed (m×n) number of pixel cells 20 at intersections of m number of data lines DL1 to DLm and n number of scan lines SL1 to SLn. The constant current source 21 is realized as two or more switching devices and a current mirror including the current source. The constant current source 21 synchronized with scan pulses applied to the scan lines SL1 to SLn in accordance with input data applies the constant current to the data lines DL1 to DLm. The switching devices 22 and 23 are realized as transistor devices such as MOS-FET. The switching devices 22 and 23 connected to the scan lines SL1 to SLn sequentially apply negative scan voltages to the scan lines SL1 to SLn to select the scan line where data are displayed. To this end, the switching devices 22 connected to the ground voltage source GND are turned on in response to a control signal T1 to apply the ground voltage GND to the selected scan line, and the switching devices 23 connected to the scan high voltage source Vhigh is turned on in response to a control signal T2 to apply the scan high voltage Vhigh to an unselected scan line.
FIG. 3 represents scan pulses applied to the scan lines SL1 to SLn, and data pulses applied to the data lines applied to the data lines DL1 to DLm.
Referring to FIG. 3, scan pulses SCAN are sequentially applied as negative voltages, i.e., forward voltage, to the scan lines SL1 to SLn, and data pulses DATA synchronized with the scan pluses SCAN are applied as positive current to the data lines DL1 to DLm. At this moment, light is emitted only at the pixel cells DATA to which the positive current is applied in accordance with the data among the pixel cells DATA connected to the scan lines SL1 to SLn to which the negative voltage is applied.
On the other hand, charges of reverse direction are charged in both ends of the pixel cell 20 connected to the unselected scan line. In such a state, if the scan line is selected when the negative voltage is applied to the unselected scan line, the pixel cells 20 charged with the reverse charges takes a considerable delay time Δt for being charged to a desired positive data current level as in a data RDATA applied to an actual EL panel of FIG. 4. This is because the input current applied to the pixel cells 20 charged with the reverse charges is wasted by the reverse charge.
The data delay of the organic EL display device can be explained more particularly through Formula 1. When the equivalent capacitance of the pixel cell 20 is C, the voltage charged in the pixel cell 20 is V, the amount of charges charged in the pixel cell 20 is Q, and the current inputted to the pixel cell 20 is I, the charge amount charged in the pixel 20 is determined as in the following Formula 1.Q=C×V=I×t  [FORUMULA 1]
If the current is uniform in accordance with time, the time t taken to charge the pixel cell 20 to a desired voltage is (C×V)/I. For example, if C is 2.4 [nF] and I is 200 [ ], the time taken to charge the pixel cell 20 to 10[V] is (2.4 [nF]×10[V])/200 [μA]=120 [μs]. Such a charging time is a considerably long time as compared with the light-emission time of a scan line in the organic EL display device.
Such a delay time deteriorates the effective response speed and brightness of the pixel cells 20. In order to compensate the deterioration of the response speed, the current should be increased, but it causes another problem of increasing power dissipation to occur because the driving voltage of each pixel 20 should be increased.
Further, in the driving apparatus of the EL display device of the relate art, the brightness between the data lines DL1 to DLm is difficult to make uniform because the data lines DL1 to DLm is driven by the constant current source 21. In order to make the brightness between the data lines DL1 to DLm uniform, the current applied to each data line DL1 to DLm must be the same. To this end, it is required to minimize the current deviation scope of a plurality of data driving integrated circuits IC each including the constant current source 21. For example, the current deviation scope of each data driving IC must be limited to within 50±0.5 [μA] for making the brightness of each data lines DL1 to DLm uniform to be about 20 [nit]. In realizing an actual circuit, designing and fabricating the data driving IC with the current deviation of within 1% not only increases the IC unit price, but also it is difficult to drive each data driving IC in within the desired current deviation even in case that the driving IC's are applied to the actual EL panel.
As a result, the related art EL display device drives the data lines DL1 to DLn by the constant current source 21 to cause the brightness and the brightness uniformity to be decreased, thereby decreasing picture quality.